1. Field of the Invention
The present invention relates to an inexpensive process which proceeds continuously with particularly high conversion rates for preparing succinic anhydride in which only extremely small amounts of the .gamma.-butyrolactone usually formed as byproduct in the hydrogenation of maleic anhydride and no mono- or hydroxycarboxylic acids having carbon numbers &lt;4 are formed.
Succinic anhydride is an important starting material for preparing thermoplastic polyesters, which have particular mechanical and chemical properties and good biodegradability.
2. Description of the Related Art
It is known to prepare succinic anhydride from succinic acid batchwise by a dehydration reaction by introducing acetic anhydride vapors into molten succinic acid (GB 507 592). It is further known to hydrogenate maleic anhydride to give succinic anhydride in a batch process over Ni (U.S. Pat. No. 2,198,153, cited in Chem. Abstracts 34 (1940), 5465.sup.9) or Pd, Rh, Pt/Al.sub.2 O.sub.3 (JP 48-7609 (1973)). It is further known to hydrogenate maleic anhydride to succinic anhydride continuously over Pd/activated carbon (SU 721 406; cited in Chem. Abstracts 93(1980), 71047 b). In addition, it is known to hydrogenate maleic anhydride to succinic anhydride over catalysts of Cu molybdate, tungstate, chromate and/or vanadate, Co molybdate, tungstate, chromate and/or vanadate and/or Ni molybdate, tungstate, chromate and/or vanadate, for which the supports used are shaped silica strands (DE 1 226 556).
The course of the reaction is shown by the following reaction scheme: ##STR1##
In the known processes for preparing succinic anhydride, batchwise suspension processes are predominantly used in which the maleic anhydride is hydrogenated with hydrogen with and without solvent over pulverulent catalysts. Batch processes have the disadvantage that their capacity is very small relative to the reaction volume, and large reaction apparatuses and storage tanks are thus required. Energy consumption and labor requirements are relatively high.
Continuous powder catalyst processes which operate with a plurality of hydrogenation reactors connected in cascade avoid some of these disadvantages. However, there is still the requirement for repeatedly specifically metering the pulverulent catalysts, circulating them by pumping and quantitatively filtering them off from the reaction product. The catalyst slurry pumps are subject to high mechanical wear. The quantitative removal of the pulverulent catalysts from the reaction product is costly. In addition, there is a high risk of relatively rapidly decreasing the catalyst activity by the additional operations. It is advantageous, therefore, to make the reaction proceed over fixed-bed catalysts. Catalysts of this type need to have a high activity which must not decrease over a prolonged period, since frequent catalyst changes in fixed-bed reactions are likewise costly. A continuous process which has been previously described is the hydrogenation of maleic anhydride to succinic anhydride over Pd/activated carbon or Cu molybdate, tungstate, chromate and/or vanadate, Co molybdate, tungstate, chromate and/or vanadate and/or Ni molybdate, tungstate, chromate and/or vanadate on silica supports (DE 1 226 556). These catalysts had only a restricted life. In addition, the reaction can generally only be carried out using a solvent.
In addition, it has been found that maleic anhydride can be hydrogenated to succinic anhydride over support-free shaped bodies which are arranged in the fixed bed and comprise oxygen-free metal powders of one or more elements of the iron subgroup of group VIII of the Periodic Table of the Elements (Mendeleev), for which it can be useful to alloy the metals of the iron subgroup with activating elements of transition group VI of the Periodic Table. In this process the powders used for preparing the shaped bodies can additionally include small proportions of non-catalytic elements (eg. silicon, aluminum, titanium, carbon) (EP 722 924 A). However, the conversion rates at space velocities of 0.14-0.4 kg of maleic anhydride per 1 of catalyst.multidot.h are very low.